In response to a recent sophistication of vehicles, seats for the vehicles having more additional functions than ever have been developed. One example is the seat for the vehicle having a side support apparatus, where a support member is accommodated inside a side support portion of a seatback. The support member, which is actuated by an electric motor or the like, pushes the sides of an occupant seated on the seat and supports the occupant from both sides. Consequently, the occupant is stably supported in the seat against a lateral load during cornering of the vehicle.
A known side support apparatus of a seat for a vehicle is disclosed in JP2005-206027A as shown in FIG. 11. FIG. 11 shows a cross section of the side support portion provided at a right-hand end portion of the seatback. In FIG. 11, a left-hand direction refers to a front of the seat for the vehicle. A side support portion having the same structure as that shown in FIG. 11 is provided at a left-hand end portion of the seatback, which is not illustrated. A side support apparatus 100 disclosed in JP2005-206027A includes a pair of brackets 103, 103 and a support plate 104 engaging with the pair of brackets 103, 103. The brackets 103, 103 are provided at a front face of a side frame 102 supporting a seatback 101 in a manner that the brackets 103, 103 face each other in a vertical direction of the seat.
Each bracket 103 includes a pair of engaging holes 103a, 103b. An engaging pin 104a provided at a lower portion of the support plate 104 is inserted into the engaging holes 103a, 103a, and an engaging pin 104b provided at an upper portion of the support plate 104 is inserted into the engaging holes 103b, 103b. Consequently, the support plate 104 is movably supported by the brackets 103, 103. An electric motor 106 is connected, via a transmission mechanism 105 including a screw and the like, to the engaging pin 104a that is inserted into one of the engaging holes, 103a, 103b. As the electric motor 106 operates, the engaging pin 104a is pulled approximately in a length direction of the engaging hole 103a (indicated by an arrow B in FIG. 11), and thus the support plate 104 moves along the engaging holes 103a, 103b. 
A front face of the support plate 104 is attached to a back face of a side pad 107a that is included in a side support portion 107. Accordingly, when the support plate 104 moves along the engaging holes 103a, 103b, the side pad 107a is pivoted about a point C relative to a top board 108 of the seatback 101 (refer to FIG. 12). A pair of the side pads 107a, 107a, that is, the side pads 107a, 107a provided on the left and right sides of the seatback, is pivoted as described above and supports an upper body of the occupant in the seat from both sides. The point C refers to a front end on a boundary between the side support portion 107 and the top board 108.
In the known side support apparatus 100 shown in FIG. 11, the engaging holes 103a, 103b are formed into arc shapes whose centers correspond to the point C because the side pad 107a is pivoted about the point C. Accordingly, in case a pivot angle of the side pad 107a needs to be increased, the engaging holes 103a, 103 need to be formed into longer arc shapes, which leads to increased size of the side support apparatus 100. Considering that a dimension of the seatback 101 in a width direction is limited, it may be difficult to accommodate the side support apparatus 100 inside the seatback 101 when the pivot angle of the side pad 107a is too large.
Meanwhile, it is considered that the pivot angle of the side pad 107a may be increased without increasing the dimension of the seatback 101 of the side support apparatus 100 in the width direction by providing the engaging hole 103b, which is positioned in a direction of a pivoting movement of the side pad 107a, in a manner that the engaging hole 103b extends in a front-rear direction of the seat (refer to FIG. 13). Consequently, a front end of the support plate 104 relative to the direction of the pivoting movement is pivoted forward, and thus the pivot angle of the side pad 107a is significantly increased. The transmission mechanism 105, and the top board 108 of the seatback 101 are not illustrated in FIG. 13.
On the other hand, an angle formed by a lengthwise direction of the engaging hole 103b, which is positioned in the direction of the pivoting movement of the side pad 107a, and by a direction in which the support plate 104 is pulled by the transmission mechanism 5 (indicated by the arrow B in FIG. 13) is increased. This causes a sliding friction between the engaging hole 103b and the engaging pin 104b, thereby making it difficult to move the support plate 104 along the engaging holes 103a, 103b smoothly. In other words, a driving force for moving the support plate 104 needs to be increased, which may lead to increased sizes of the transmission mechanism 105 and the electric motor 106.
In order to move the support plate 104 along the engaging holes 103a, 103b smoothly, a method is possible where the transmission mechanism 105 is connected to the engaging pin 104b, which is positioned in the direction of the pivoting movement of the side pad 107a, so as to push the engaging pin 104b in the lengthwise direction of the engaging hole 103b (the front-rear direction of the seat for the vehicle). A dimension of the seatback 101 in the front-rear direction is, however, even more limited than that in the width direction. Therefore it is almost impossible to place the transmission mechanism 105 in the seatback 101 in the front-rear direction. A need thus exists for an operating mechanism of a side support apparatus and the side support apparatus using the operation mechanism, the side support apparatus adapted to be mounted on a seat for a vehicle, which is not susceptible to the drawback mentioned above.